1. Field of the Invention
This invention relates to a photosensitive resin composition which is a halogen-free formulation imposing little burden on the environment and is capable of forming a solder resist which is flame-retardant, exhibits little warpage and excels in bending characteristics. This invention further relates to a dry film and a flame-retardant printed wiring board which use such a photosensitive resin composition.
2. Description of the Prior Art
Heretofore, flame retardancy is required for a printed wiring board and a flexible wiring board (hereinafter abbreviated as “FPC”) because they are mounted in electronic devices, and the flame retardancy is also required for solder resists which are parts thereof. Among these wiring boards, the FPC usually uses a polyimide substrate which is, unlike a printed wiring board of a glass-epoxy substrate, a thin film. However, since the thickness of the solder resist which should be applied to the printed wiring board is the same as that to the FPC, in the case of the thin FPC the burden of flame retardancy to the solder resist becomes large relatively.
Therefore, various proposals for imparting flame retardancy to the solder resist have been made hitherto. For example, JP 2007-10794A proposes a flame-retardant photosensitive resin composition for FPC, which composition comprises (a) a binder polymer, (b) a photopolymerizable compound having a halogenated aromatic ring, such as a bromophenyl group, and a polymerizable ethylenically unsaturated bond, such as an acryloyl group, in its molecule, (c) a photopolymerization initiator, (d) a blocked isocyanate compound, and (e) a phosphorus-containing compound having a phosphorus atom in its molecule. However, the use of a halogenated compound such as a compound having a halogenated aromatic ring and a polymerizable unsaturated double bond is not preferred from the viewpoint of the environmental impact.
As for the printed circuit board of the thin film represented by the FPC, the occurrence of warpage due to cure shrinkage at the time of photo-curing or thermal curing of a solder resist becomes a problem.
On the other hand, as a halogen-free flame-retardant solder resist, JP 2007-41107A, for example, proposes a photosensitive resin composition which is soluble in an aqueous alkaline solution and comprises a resin, which is soluble in an aqueous alkaline solution and produced by the addition reaction of a polybasic acid anhydride to a reaction product of a biphenyl novolak type epoxy resin and an unsaturated monocarboxylic acid, a biphenyl novolak type epoxy resin as a curing agent, and a photopolymerization initiator. However, this composition is not enough to reduce warpage and is insufficient in flexing properties.